SpannersatCX has nailed it.
Forget about the type of gyro's and whether it has built in GPS and/or Air Data capabilities. These are irrelevant to the question.
INS computes a navigation solution whereas IRS only supplies present position to a separate navigation system (FMS, Area Nav, Mission Computer, etc).
There is nothing more to it.
Some L1011's and DC10's originally had Litton 58 mechanical gyro IRS units feeding an Area Nav system. Others had INS's without the Area Nav. Purely a customer option.